For example, a radiator fan mounted in a vehicle is controlled so that the number of rotations is controlled according to a temperature of engine cooling water and when the temperature of engine cooling water is low, the number of rotations is decreased in order to reduce noise and when the temperature of engine cooling water becomes high, the number of rotations is increased in order to enhance a cooling effect (for example, Patent Reference 1).
FIG. 7 is a circuit diagram showing a conventional example of a load control circuit for driving a motor M1 for radiator fan driving. As shown in FIG. 7, the motor M1 is connected to a battery VB through an electronic switch (T101) such as a MOSFET. Also, the load control circuit includes an engine computer 101, a pulse generator 102 and a driver 103, and when a rotation command signal of the motor M1 according to a temperature of engine cooling water is output from the engine computer 101 to the pulse generator 102, a PWM signal with a desired duty ratio is generated by the pulse generator 102. That is, when the temperature of engine cooling water is low, a PWM signal with a short on time (that is, a low duty ratio) is generated and when the temperature of engine cooling water is high, a PWM signal with a long on time (that is, a high duty ratio) is generated.
Then, the driver 103 outputs the PWM signal generated by the pulse generator 102 to a control terminal (a gate in the case of the MOSFET) of the electronic switch (T101).
As a result of that, the electronic switch (T101) performs an on-off action by the supplied PWM signal and electric power is supplied to the motor M1 and the motor M1 is rotated by a desired number of rotations. That is, the motor M1 is rotated and driven by the number of rotations according to a temperature of engine cooling water.
Here, a frequency of a PWM signal output from the pulse generator 102 is set at a high frequency of about 19 [KHz] greater than a human audio frequency. Also, when the frequency of the PWM signal is set high, a heating value generated for a transitional period at the time when the electronic switch (T101) performs an on-off action becomes large, so that the electronic switch (T101) is stored inside a unit having a radiation fin. Further, the radiation fin is installed in a place with good air permeability.    Patent Reference 1: JP-A-2005-80384